Implantable medical devices (IMDs) have many functions including the delivery of therapies to cardiac patients, neuro-stimulators, muscular stimulators, and others. Application of the present invention is described below in the context of implantable cardiac pacemakers and/or defibrillators, it being understood that the principles herein may have applicability to other implantable medical devices as well.
An example of an implantable medical device (IMD) includes a device commonly referred to as a pacemaker, which is used to stimulate the heart into a contraction if the sinus node of the heart is not properly timing, or pacing, the contractions of the heart. Modern implantable medical devices also perform many other functions beyond that of pacing. For example, a pacemaker/cardioverter/defibrillator (PCD) performs therapies such as defibrillation and cardioversion as well as providing several different pacing therapies, depending upon the needs of the user and the physiologic condition of the user's heart.
In typical use, a PCD is implanted in a convenient location usually under the skin of the user and in the vicinity of the one or more major arteries or veins. One or more electrical leads connected to the PCD are inserted into or on the heart of the user, usually through a convenient vein or artery. The ends of the leads are placed in contact with the walls or surface of one or more chambers of the heart, depending upon the particular therapies deemed appropriate for the user.
One or more of the leads is adapted to carry a current from the PCD to the heart tissue to stimulate the heart in one of several ways, again depending upon the particular therapy being delivered. The leads are simultaneously used for sensing the physiologic signals provided by the heart to determine when to deliver a therapeutic pulse to the heart, and the nature of the pulse, e.g., a pacing pulse or a defibrillation shock.
In the sensing mode, sense amplifiers coupled to the leads provide amplification to electrogram signals picked up by the sensing electrodes in the heart. The analysis of these signals by the PCD determines whether a therapy (a pacing pulse or a defibrillator shock) should be administered. If erroneous signals are detected by the PCD, an unnecessary therapy may be administered, providing an unnecessary pacing pulse or defibrillator shock to the patient. One cause of erroneous interpretation of sensing signals is oversensing, that is, the false detection of a depolarization event.
Sensing can be accomplished in a number of ways. If two bipolar leads are used, one lead is typically placed within the right ventricle of the heart and a second lead is placed within the right atrium of the heart. Both leads include two sensing elements: a tip electrode that is attached to the wall or surface of the heart, and a ring electrode that is located on the lead but removed some distance from the tip electrode. A high voltage coil located on one or both of the leads can also be used for sensing, as can the implanted PCD can itself. Some cardiac conditions require sensing at both the right ventricle and the right atrium, and still others may add sensing at the left ventricle via a third lead positioned within the coronary sinus. As a result, there are a large number of sensing paths and combinations available for use, depending upon the configuration and programming of the specific implantable device, and some PCDs can be configured to switch to an alternate sensing path if the primary path is determined to be faulty.
Oversensing of cardiac waves may be caused by lead fractures (e.g., conductor break, insulation break, adaptor failure), connectors (e.g. loose set screw), T-wave oversensing, R-wave oversensing, electromagnetic interference (EMI), and myopotentials. In the past, oversensing problems (e.g., myopotentials, T-wave oversensing) have been dealt with by modifying sense amplifiers, filters and PCD lead electrodes. In currently used PCDs the sense amplifiers have self adjusting sensing thresholds for sensitivity, so oversensing often occurs as a result of lead failure. Upon detection of an R-wave, the threshold of the sense amplifier is raised to about 75% of the R-wave. The sense amplifier threshold then decays until the next R-wave is sensed. In this manner the sensing threshold of the sense amplified is continually adjusted to allow for variations in the sensed strength of the R-wave. However, there is still room for improved techniques for eliminating the effects of oversensing.
Accordingly, it is desirable to provide an additional mechanism for dealing with oversensing of depolarization events. In addition, it is desirable to provide an algorithm to be incorporated into detection algorithms in the IMD to prevent the detection and erroneous application of therapies for detected episodes caused by oversensing. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.